Method and apparatus for enhancing signal strength for improved generation and placement of model-based sub-resolution assist features (MB-SRAF)

ABSTRACT

Model-Based Sub-Resolution Assist Feature (SRAF) generation process and apparatus are disclosed, in which an SRAF guidance map (SGM) is iteratively optimized to finally output an optimized set of SRAFs as a result of enhanced signal strength obtained by iterations involving SRAF polygons and SGM image. SRAFs generated in a prior round of iteration are incorporated in a mask layout to generate a subsequent set of SRAFs. The iterative process is terminated when a set of SRAF accommodates a desired process window or when a predefined process window criterion is satisfied. Various cost functions, representing various lithographic responses, may be predefined for the optimization process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.13/007,358 (Now U.S. Pat. No. 8,443,312), filed Jan. 14, 2011, whichclaims the benefit of U.S. Provisional Application Ser. No. 61/295,100,filed Jan. 14, 2010, all of which are incorporated herein by referencein their entirety.

FIELD

This invention relates generally to resolution enhancement techniquesfor photolithography and relates more particularly to a system andmethod for model-based sub-resolution assist feature generation andmanipulation.

BACKGROUND

The integrated circuit industry has, since its inception, maintained aremarkable growth rate by driving increased device functionality atlower cost. One of the primary enabling factors of this growth has beenthe ability of optical lithography to steadily decrease the smallestfeature size that can be formed as part of the integrated circuitpattern. The steady decline in feature size and cost and thecorresponding increase in the density of features printed per circuitare commonly referred to as “Moore's Law” or the lithography “roadmap.”

The lithography process involves creating a master image on a mask orreticle (mask and reticle are used interchangeably herein), thenprojecting an image from the mask onto a resist-covered semiconductorwafer in order to create a pattern that matches the design intent ofdefining functional elements, such as transistor gates, contacts etc.,on the wafer. The more times a master pattern is successfully replicatedon a wafer within the design specifications, the lower the cost perfinished device or “chip” will be. Until recently, the mask pattern hasbeen an almost exact duplicate of the desired pattern at the waferlevel, with the exception that the mask level pattern may be severaltimes larger than the wafer level pattern, due to an imaging reductionratio of the exposure tool. The mask is typically formed by depositingand patterning a light absorbing material on quartz or anothertransparent substrate. The mask is then placed in an exposure tool knownas a “stepper” or “scanner” where light of a specific exposurewavelength is directed through the mask onto the wafers. The light istransmitted through clear areas of the mask, but is attenuated by adesired amount, typically between 90 and 100%, in the areas covered bythe absorbing layer. The light that passes through some regions of themask may also be phase shifted by a desired phase angle, typically aninteger multiple of 90 degrees. After being collected by the projectionoptics of the exposure tool, the resulting aerial image pattern is thenfocused onto the wafers. A light-sensitive material (photoresist orresist) deposited on the wafer surface interacts with the light to formthe desired pattern on the wafer, and the pattern is then transferredinto the underlying layers on the wafer to form functional electricalcircuits according to well-known processes.

In recent years, the feature sizes being patterned have becomesignificantly smaller than the wavelength of light used to transfer themask pattern onto the wafer. This trend towards “sub-wavelengthlithography” has resulted in increasing difficulty in maintainingadequate process margins in the lithography process. The aerial imagescreated by the mask and exposure tool lose contrast and sharpness as theratio of feature size to wavelength decreases. This ratio is quantifiedby the k₁ factor, defined as the numerical aperture (NA) of the exposuretool times the minimum feature size W_(f) divided by the wavelength λ,i.e., k₁=NA·W_(f)/λ. There is limited practical flexibility in choosingthe exposure wavelength, while the numerical aperture of exposure toolsis approaching physical limits. Consequently, the continuous reductionin device feature sizes requires more and more aggressive reduction ofthe k₁ factor in lithographic processes, i.e. imaging at or below theclassical resolution limits of an optical imaging system.

New methods to enable low-k₁ lithography have used master patterns onthe mask that are no longer exact copies of the final wafer levelpattern. The mask pattern is often adjusted in terms of the size andplacement of pattern features as a function of pattern density or pitch.Other techniques involve the addition or subtraction of extra corners onthe mask pattern (“serifs,” “hammerheads,” and other patterns) known asOptical Proximity Correction, or OPC; and the addition of othergeometries that are not intended to be replicated on the wafer at all.The sole purpose of these non-printing “assist features,” also known asSub-Resolution Assisting Features (SRAFs) or scattering bars, is toenhance the printability of the “main features.” The SRAFs are typicallysmall bars (the term “bar” encompasses lines and other geometric shapes)placed close to the main features so that the printability of the mainfeatures is more robust against focus and/or dose change. All of thesemethods are often referred to collectively as Resolution EnhancementTechnology (RET). With decreasing k₁, the magnitude of proximity effectsincreases dramatically. In current high-end designs, more and moredevice layers require RET, and almost every feature edge requires someamount of adjustment to ensure that the printed pattern will reasonablyresemble the design intent. The implementation and verification of suchextensive RET application is only made possible by detailed full-chipcomputational lithography process modeling, and the process is generallyreferred to as model-based RET.

The cost of manufacturing advanced mask sets is steadily increasing.Currently, the cost has already exceeded one million dollars per maskset for an advanced device. In addition, the turn-around time is alwaysa critical concern. As a result, lithography-driven RET design, whichassists in reducing both the cost and turn-around time, has become anintegral part of semiconductor manufacturing.

As the lithography process entered below the 65 nm node (such as, 28 nmnode), leading-edge chip designs have minimum feature sizes smaller thanthe wavelength of light used in advanced exposure tools. SRAFs becomeindispensable even if OPC techniques provide good results. Typically,OPC will modify the design layout so that a resist image (RI) contour isclose enough to the design target at nominal condition. However, theProcess Window (PW) is rather small without any extra features. SRAFsare needed to enhance the printability of the main features across awider range of defocus and delta dose scenarios in order to maintainadequate process margins in the lithography process.

One method for implementing SRAFs that is widely in use is rule-basedSRAF placement using an empirical (manual) rule-generator. In thismethod, a combination of benchmark test patterns with different SRAFconfigurations are printed (or simulated) on a wafer. Critical Dimension(CD) is then measured on the wafer, a set of rules for SRAF placement isdrawn from the CD comparison, and finally the set of rules is used inSRAF placement for each main feature segment in a design. It should benoted that empirical rule-based SRAF placement requires an efficientmechanism to solve many conflicts between SRAFs derived from differentmain feature segments.

Another proposed method to generate SRAFs is based on inverselithography techniques. In this method, the goal is to identify a maskimage that minimizes an objective function (also referred to as a “costfunction”). The objective function includes the difference between theresulting aerial image and the ideal design target image and also thedifference between the aerial image intensity at the design target edgelocations and the threshold for contours across wide ranges of defocusand delta dose conditions. To solve this non-linear programming problem,various iterative approaches are used to identify a local minimumsolution.

While these methods have demonstrated some successes, theirdisadvantages have slowed the development cycle and limited their usage.For example, the empirical (manual) rule-generator has the followingdrawbacks: unable to take into account all possible patterns/spaces/linewidths in a limited number of test patterns; high cost and low speed tomanufacture the mask, print the wafer, and measure CD; difficulty inmeasuring the SRAFs' performance across the PW; and difficulty inresolving SRAF conflicts. The inverse lithography based method is alsocomplicated and slow, since it may require quite a few slow iterationsto converge. It may also converge to a local optimum, and it is notfeasible to use it directly as it generates continuous values for eachpixel while only rectangular shaped patterns with mask constraints aremanufacturable. In addition, the objective function includes thedifference between the whole aerial image and the design target, whilein practice, the fidelity of the aerial image contours is of moreinterest. The focus on pixels deep inside or outside main features maybe counterproductive.

Computer models have been created to come up with a faster and efficientSRAF placement algorithm that takes 2D pattern shapes into considerationand optimizes for a desired PW. This technique is called Model-BasedSub-Resolution Assist Feature (MB-SRAF) method. MB-SRAF methods havebeen exercised as the RET solution for certain applications, such as,for printing trench contacts, vias, and metal layers for 28 nmtechnology node.

Current MB-SRAF algorithms depend on signal mapping (i.e. measuringsignal strength at various locations) to guide SRAF placements. Thesignal map, known as SRAF Guidance Map (SGM), is derived from variantsof image contrast and process focus derivatives. Details of thegeneration of an SGM can be found in co-pending U.S. patent publicationno. 2008/0301620, which is incorporated herein by reference. The currentMB-SRAF methods are based on an initial SGM, which may not be optimizedfor a process window. There is a need for a method that can dynamicallyoptimize the SGM, and can accommodate a large enough process window,while reducing the computational load.

SUMMARY

Embodiments of the present invention provide methods and systems forModel-Based Sub-Resolution Assist Feature (MB-SRAF) generation andplacement. According to an aspect of the invention, signal strength ofan SRAF guidance map (SGM) is iteratively enhanced to finally output anoptimized set of SRAFs. An optimized “set” of SRAFs may include one ormore SRAFs. SRAF polygons generated in a prior round of iteration areincorporated in an altered or unaltered mask layout to generate asubsequent updated SGM that is used to generate a subsequent set ofSRAFs. The iterative process is terminated when a set of SRAFaccommodates a desired process window (PW) or when a predefined PWcriterion is satisfied. A cost function, representing a lithographicresponse, is predefined for the optimization process.

In one embodiment, iterations use progressively updated SGM and originalmask layout to boost image signal for accurate SRAF placement. Inanother embodiment, iterations use progressively updated SGM andprogressively updated mask layout with Optical Proximity Correction(OPC) for possible further improvement in signal strength and SRAFplacement.

According to another aspect of the invention, a computer program productis disclosed that enables a computer to execute the above MB-SRAF signalboosting and SRAF placement optimization methods.

According to yet another aspect of the invention, a method is disclosedfor enhancing signal strength for placing sub-resolution assist features(“SRAF”) with respect to one or more target patterns in a mask layout.The method comprises: generating an initial SRAF guidance map for themask layout; placing a first set of one or more SRAF in the mask layoutaccording to the initial SRAF guidance map; altering one or more ofnumber, location and dimension of at least a portion of the first set ofone or more SRAFs; measuring improvement in signal strength in theinitial SRAF guidance map; and generating an updated SRAF guidance mapwith enhanced signal strength.

Embodiments of the present invention provide an alternative fastersolution than other computation-intensive techniques, such as, InverseLithography Technology (ILT) by improving the existing SGM-based SRAFgeneration method.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1A is an exemplary block diagram illustrating a typicallithographic projection system.

FIG. 1B is an exemplary block diagram illustrating the functionalmodules of a lithographic simulation model.

FIG. 2A schematically depicts an initial SGM intensity map generatedcorresponding to a “bulleye” illumination source with a square contactpad target pattern.

FIG. 2B schematically depicts the optimal SGM intensity map after themethod of this invention is applied corresponding to the “bulleye”illumination source with the square contact pad target pattern.

FIGS. 3A-B illustrate improvement in SRAF signal strength and SRAFplacement accuracy for a square contact pad target pattern, according toan embodiment of the invention. FIG. 3A shows an example of SRAFplacement before the methods of the invention are applied, and FIG. 3Bshows an example of improved SRAF placement after the methods of theinvention are applied.

FIG. 4 illustrates improved lithographic response using SRAF for asquare contact pad target pattern, according to an embodiment of theinvention.

FIGS. 5-6 illustrate improvement in lithographic process window usingembodiments of the invention.

FIGS. 7 and 8A-B show flowcharts illustrating example process steps,according to various embodiments of the invention.

FIGS. 9A-E show a set of SRAF solutions generated by different rounds ofiterations, according to embodiments of the invention.

FIG. 10 illustrates depth of focus (DOF) values associated with variousSRAF solutions, according to embodiments of the invention.

FIGS. 11A-B and 12 illustrate improvement in SRAF signal strength andSRAF placement accuracy for an example mask layout, according to anembodiment of the invention.

FIG. 13 illustrates an array of target patterns with correspondingsymmetric or asymmetric SRAFs, according to an embodiment of the presentinvention.

FIG. 14 is a block diagram that illustrates a computer system which canassist in the implementation of the simulation method of the presentinvention; and

FIG. 15 schematically depicts a lithographic projection apparatussuitable for use with the method of the present invention.

DETAILED DESCRIPTION

The present invention will now be described in detail with reference tothe drawings, which are provided as illustrative examples of theinvention so as to enable those skilled in the art to practice theinvention. Notably, the figures and examples below are not meant tolimit the scope of the present invention to a single embodiment, butother embodiments are possible by way of interchange of some or all ofthe described or illustrated elements. Moreover, where certain elementsof the present invention can be partially or fully implemented usingknown components, only those portions of such known components that arenecessary for an understanding of the present invention will bedescribed, and detailed descriptions of other portions of such knowncomponents will be omitted so as not to obscure the invention.Embodiments described as being implemented in software should not belimited thereto, but can include embodiments implemented in hardware, orcombinations of software and hardware, and vice-versa, as will beapparent to those skilled in the art, unless otherwise specified herein.In the present specification, an embodiment showing a singular componentshould not be considered limiting; rather, the invention is intended toencompass other embodiments including a plurality of the same component,and vice-versa, unless explicitly stated otherwise herein. Moreover,applicants do not intend for any term in the specification or claims tobe ascribed an uncommon or special meaning unless explicitly set forthas such. Further, the present invention encompasses present and futureknown equivalents to the known components referred to herein by way ofillustration.

General Environment in a Lithographic System for Implementing ExampleEmbodiments of the Present Invention

Prior to discussing the present invention, a brief discussion regardingthe overall simulation and imaging process to be calibrated is provided.FIG. 1A illustrates an exemplary lithographic projection system 10. Themajor components are a light source 12, which may be a deep-ultravioletexcimer laser source, illumination optics which define the partialcoherence (denoted as sigma) and which may include specific sourceshaping optics 14, 16 a and 16 b; a mask or reticle 18; and projectionoptics 16 c that produce an image of the reticle pattern onto the waferplane 22. An adjustable filter or aperture 20 at the pupil plane mayrestrict the range of beam angles that impinge on the wafer plane 22,where the largest possible angle defines the numerical aperture of theprojection optics NA=sin(Θ_(max)).

In a lithography simulation system, these major system components aredescribed by separate functional modules, for example, as illustrated inFIG. 1B. Referring to FIG. 1B, the functional modules include the designlayout module 26, which defines the target design; the mask layoutmodule 28, which defines how the mask is laid out using the targetdesign to be utilized in the imaging process; the mask model module 30,which models the properties of the physical mask to be utilized duringthe simulation process; the optical model module 32, which defines theperformance of the optical components of lithography system; and theresist model module 34, which defines the performance of the resistbeing utilized in the given process. As is known, the result of thesimulation process produces, for example, predicted contours and CDs inthe result module 36.

More specifically, it is noted that the properties of the illuminationand projection optics are captured in the optical model module 32 thatincludes, but is not limited to, NA-sigma (σ) settings as well as anyparticular illumination source shape, where σ (or sigma) is the innerand/or outer radial extent of the illuminator. The optical properties ofthe photo-resist layer coated on a substrate—i.e. refractive index, filmthickness, propagation and polarization effects—may also be captured aspart of the optical model module 32.

Finally, the resist model module 34 describes the effects of chemicalprocesses which occur during resist exposure, PEB and development, inorder to predict, for example, contours of resist features formed on thesubstrate wafer. The objective of the simulation is to accuratelypredict, for example, edge placements and critical dimensions (CDs),which can then be compared against the target design. The target designis generally defined as the pre-OPC mask layout, and will be provided ina standardized digital file format, such as GDSII or OASIS.

In general, the connection between the optical and the resist model is asimulated aerial image within the resist layer, which arises from theprojection of light onto the substrate, refraction at the resistinterface and multiple reflections in the resist film stack. The lightintensity distribution (i.e. aerial image intensity) is turned into alatent “resist image” by absorption of photons, which is furthermodified by diffusion processes and various loading effects. Efficientsimulation methods that are fast enough for full-chip applicationsapproximate the realistic 3-dimensional intensity distribution in theresist stack by a 2-dimensional aerial (and resist) image.

As discussed in the background section, this invention updates the masklayout module 28 by optimizing an SRAF Guidance Map (SGM).

One method of generating an SGM includes: computing an image gradientmap of the mask layout, and for each field point in the mask layout,computing a contribution of a hypothetical unit source placed at thefield point. Each pixel in an SGM is assigned a value that representsthe computed contribution of the hypothetical unit source. Each pixelvalue indicates whether the pixel would contribute positively to edgebehavior of target patterns in the mask layout if the pixel is includedas part of an SRAF. SRAFs are placed in the mask layout according to thegenerated SGM. The mask layout may be pre-OPC or post-OPC. Optionally,pre-defined SRAF placement rules may be followed in placing the SRAFsaccording to the SGM. Further details of SGM generation and SRAFplacements can be found in co-pending U.S. patent publication no.2008/0301620, which is incorporated herein by reference. The presentinvention discloses methods for iteratively optimizing signal strengthof an SGM for improving SRAF placements.

FIG. 2A shows an SGM 200A generated with a “bull's eye” illuminationsource aligned with the center point of an example pre-OPC squarecontact pad target pattern 202, represented by the dotted outlines. SGM200A is the initial SGM before the techniques of the present inventionare applied. The dimension of the square pattern 202 can be, forexample, 60 nm. However the invention is not limited by the dimension orshape of the target pattern in any way. Usually, SRAFs are placed in theannular region 204 with comparatively higher intensity. FIG. 2Billustrates an optimized SGM 200B after the techniques of the presentinvention are applied. Additional SRAFs may be placed in the newlydefined high intensity region 206 identified by the application of thepresent invention. Additionally, signal strength of the otherhigh-intensity annular regions are improved by the application of thepresent invention.

Example Embodiments of the Present Invention

FIG. 3A shows a set of example SRAFs, non-printing scatter bars 304A-D,placed in the vicinity of the target patterns 202. In the example shownin FIG. 3A, the scatter bars 304A-D are at a distance d1 away from thetarget pattern 202. The layout 300A in FIG. 3A is typically generated byan existing simulation method to generate an SGM.

FIG. 3B shows a layout 300B, generated by the improved SGM generationalgorithm of the present invention. Layout 300B includes a different setof SRAFs for the same target pattern 202. The simulation location isrepresented by a cutline 303 passing through the center point of thetarget pattern 202. SRAFs generated by simulation in the layout 300Bincludes an inner set of scatter bars 310A-D, which are placed adistance d2 away from the target pattern 202, in addition to the outerset of scatter bars, 308A-D, which may be placed at a distance d1′,which may or may not be similar to d1. In an example, d1 may be 157 nm,and d2 may be 83 nm, though the scope of the invention is not limited bythese example values of d1, d1′, and d2. The dimension of scatter bars308A-D in FIG. 3B may or may not be similar to the dimensions of scatterbars 304A-D of FIG. 3A. Addition of the scatter bars 310A-D improves theimaging of the target pattern 202.

FIG. 4 shows signal profiles 401-404 along the cutline 303 plotted as afunction of x-coordinate for the target pattern 202. The signal mayrepresent a local intensity slope value. Other signal values may be usedtoo, such as a local intensity slope normalized by critical dimension,inverse intensity slope, intensity divided by local slope, intensitydivided by square of the local slope, process focus derivative signal,etc. The central dips in the signal profiles 401-404 are aligned withthe center point of the target pattern 202. Signal profile 401corresponds to an initial SGM, generated from the original pre-OPC masklayout. In the region 410 within the dashed outline, signal profile 401does not show an easily discernable gradient, which is sufficient toidentify a possible ridge location for SRAF placement. Therefore, usingonly the initial SGM, a lithographic simulation is not likely to placeany SRAF in the region 410. Signal profile 401 may result in mask layout300A shown in FIG. 3A. However, using the iterative algorithm of thepresent invention, signal strength and signal gradient in the region 410are improved progressively. Profiles 402, 403, and 404 correspond to2^(nd), 3^(rd), and 4^(th) SGMs respectively, which are results ofincreasing number of optimizing iterations. Profile 404 clearly shows adiscernable dip in signal within region 410. Therefore, it will bebeneficial to place SRAFs in the region 410. Signal profile 404 mayresult in mask layout 300B shown in FIG. 3B, where additional SRAFscatter bars 310A-D are placed at the mask layout coordinates includedin the region 410.

FIG. 5 graphically illustrates how example embodiments of the presentinvention accommodate better process window as a result of improvedgeneration and placement of SRAFs around a target pattern 202, whencompared to the existing simulation methods in the art. The X-axis andY-axis in FIG. 5 represent Degree of Freedom (DOF) values and ExposureLatitude (EL) percentage values, respectively. The dashed curve 504represents results from the existing art simulation method, while thesolid curve 502 represents results from the simulation method of thepresent invention. As indicated in FIG. 5, the present inventionimproves the DOF by ‘e’), which accounts for DOF improvement. In theexample shown in FIG. 5, ‘e’ is 71 nm, corresponding to 68% improvementin DOF. In other words, generating a more comprehensive set of SRAFs andimproving SRAF placement in a mask layout according to the embodimentsof the present invention significantly improve process window in alithographic process.

FIG. 6 shows a plot of image log slope (ILS) value as a function ofx-coordinate along the cutline 303 shown in FIG. 3B. The (0,0) point ofthe plot in FIG. 6 is the center point of the target pattern 202. FIG. 6shows that when no SRAF is placed (i.e., neither an existing SRAFgeneration algorithm is used, nor the SRAF generation algorithm of thepresent invention is used), the ILS values (along curve 603) are lowerthan the ILS values (along curve 604) obtained with SRAFs placedaccording to an algorithm of the present invention. Improved ILSindicates improved image contrast. ILS can be used as a possibleobjective function or cost function that is used in the iterativeoptimization process of the present invention. Other possible costfunctions may include, an edge placement error (EPE) value, a Mask ErrorEnhancement Factor (MEEF) value, or any possible combination of ILS, EPEand MEEF, etc.

FIG. 7 shows a flowchart that illustrates example steps of a method 700for improved generation and placement of SRAFs, according to anembodiment of the present invention. Method 700 starts at step 702,where an original mask layout is obtained. Original mask layoutcomprises the target patterns before any OPC is applied. In step 704,optionally OPC may be applied to the target patterns. In step 706, aninitial SGM is generated from the pre-OPC or post-OPC mask layout. Instep 708, a first set of SRAFs are generated based on the initial SGM.In step 710, an altered mask layout is generated, where the pre-OPC orpost-OPC target patterns and the first set of SRAFs are placedsimultaneously. In step 712, the SGM is updated based on the alteredmask layout of step 710. In step 714, a subsequent set of revised SRAFsare generated and placed in the mask layout. Steps 710, 712, and 714 maybe repeated iteratively, as shown by the path 716, until a pre-definedsimulation termination condition is satisfied.

The main difference in method 700 compared to the existing art is that,in the existing art, only an initial SGM is used, whereas, in method700, SGM is progressively updated to generate a better optimized SGMthat facilitates in generating a more comprehensive and more accuratelyplaced set of SRAFs. In generating updated SGM, not only the targetpattern geometry (pre-OPC or post-OPC) is considered, but the geometryof a prior set of SRAF polygons located in the altered mask layout isalso considered. In this manner, the signal strength of the SGM isiteratively boosted, so that a subsequent set of SRAFs are more easilyand accurately generated and placed.

Persons skilled in the art will appreciate that the progressive updatingof the SGM is done by measuring the improvement in signal strength inthe SGM. Improvement of signal strength of SGM is a result of selectingthe appropriate SRAF(s) for a target pattern. A single SRAF or multipleSRAFs may contribute to improvement in SGM signal strength. The term “aset of SRAF” encompasses one SRAF or multiple SRAFs. When a prior set ofSRAF is altered to update the SGM, the entire set of SRAF may bereplaced, or only one or a few of the SRAFs from the prior set may bealtered. The alteration of SRAFs may include alteration of dimension ofSRAFs, alteration of number of SRAFs, and/or alteration of location ofSRAFs. In an example embodiment, only location of the SRAFs may bechanged to update SGM without changing the dimension of the SRAFs. Inanother embodiment, only dimension of one or more SRAFs may be changedwithout changing the location of the SRAFs. In yet another embodiment,the number of SRAFs may be changed but location and dimension of the atleast a portion of the prior set of SRAFs are kept intact. Number ofSRAFs may be determined by the effectiveness to achieve a predefined PWcriterion and by mask manufacturing rule check (MRC) constraints. WhenSGM does not improve anymore, adding more SRAFs does not help. Again,persons skilled in the art will appreciate that the examples are forillustrative purposes only, and other possible alterations of SRAFs canbe done if improvement in the SGM is observed.

FIGS. 8A-B show two embodiments of the optimization method 700. Theflowchart in FIG. 8A shows a short-loop or partial-loop optimizationprocess 800A, and the flowchart in FIG. 8B shows a full-loop orlong-loop optimization process 800B. Process 800A and 800B only show theiterative portion of the process 700 that starts after step 708.

Process 800A starts at step 802A, where original mask layout (withoutOPC) is obtained. The SRAFs from the immediately prior iteration step,i.e. (n−1)^(th) iteration step, are also obtained. In other words, thealtered mask layout in step 802A includes original target patterns plusthe SRAFs from the (n−1)^(th) iteration.

In step 804A, SGM for the n^(th) iteration is generated based on thealtered mask layout of step 802A.

In step 806A, SRAFs are generated for the n^(th) iteration from the SGMin step 804A.

In step 808A, an altered mask layout is generated that includes SRAFsfrom step 806A, as well as the original target patterns (without OPC).

In step 810A, a lithographic response is determined using the alteredmask layout of step 808A. the lithographic response may be defined by acost function, such as an ILS, and EPE, or a combination of ILS and EPE.The lithographic responses are associated with the parameters of theprocess window. Typical process window parameters are focus, exposuredose, etc. Persons skilled in the art will appreciate that these areonly a few examples of the possible lithographic responses and processwindow parameters that can be tracked, and the scope of the invention isnot limited by the choice of lithographic response and process windowparameters.

In step 812A, it is determined whether desired process window isaccommodated (or a predefined process window criterion is met) or not bythe current altered mask layout at the end of the n^(th) iteration. Ifprocess window criterion is satisfied, then the iteration is terminated.If not, then the iteration is continued to the next iterative step, asshown by the path 816A. Persons skilled in the art will appreciate thatthe predefined process window criterion may include a situation wherethe iteration does not necessarily convolute, but still a specificpredefined PW size is accommodated.

Method 800B shown in FIG. 8B is different from the method 800A in termsof the initial mask layout that is used in step 802B. Instead of usingthe original mask layout, in step 802B, the altered mask layoutgenerated at the end of the immediately prior round of iteration, i.e.the (n−1)^(th) iteration is used to generate the SGM for the n^(th)round of iteration in step 804B.

Steps 804B and 806B are similar to steps 804A and 806A.

In step 808B, further OPC is applied to the altered mask layout from the(n−1)^(th) iteration. The corrected altered mask layout at the end ofstep 808B includes post-OPC target patterns (OPC may have been applied(n−1) times to the original target patterns) and SRAFs generated by then^(th) iteration.

Steps 810B, 812B, 814B and path 816B are similar to the correspondingsteps/paths in FIG. 8A.

Persons skilled in the art will appreciate that the methods shown inFIGS. 7 and 8A-B only depict illustrative steps. Not all the steps needto be included in every embodiment, and additional intermediate/terminalsteps may be included in the methods, as applicable. The sequence of thesteps may be altered. The method of iteratively placing a subsequent setof SRAF may comprise replacing a prior set of SRAF altogether with acompletely new subsequent set of SRAF. Alternatively, the method maycomprise retaining at least portions of a prior set of SRAF, andadjusting the prior set of SRAF to obtain the subsequent set of SRAF.

FIGS. 9A and 9B show that the iterative SRAF solution does not have tobe unique. For example, the layout 900A of FIG. 9A shows SRAF solutionwithout corner considerations, and the layout 900B of FIG. 9B shows SRAFsolution with corner consideration. Layout 900A shows edge-alignedscatter bars 905 and 906 (other similar scatter bars are not labeled forthe sake of clarity). Layout 900B shows edge aligned scatter bars 910and 911, and additional corner SRAFs 912 and 913. Note that edge alignedscatter bars 905 and 906 may be placed at different locations than edgealigned scatter bars 911 and 910. Their dimensions may differ from oneanother too.

FIG. 9D shows that in progressive rounds of iteration, location anddimension of corner SRAFs and/or edge aligned scatter bars may vary.FIG. 9D superimposes layout 900B of FIG. 9B and layout 900C of FIG. 9Cto accentuate the relative differences between the layouts 900B and900C. In FIG. 9D, the SRAFs of layout 900C (e.g., scatter bar 914,corner SRAF 916 etc.) are shown in hatched lines.

In FIG. 9E, layout 900E is shown, which shows yet additional cornerSRAFs 918 and 920, that did not appear in the prior layouts 900B or900C. Therefore, it can be understood that as a result of iterativerefinement, additional SRAFs may be generated. In some cases, some SRAFsmay be discarded too if performance improvement is achieved bydiscarding some SRAFs. Usually the iteration is terminated when thedifference in performance improvement is negligible between twosuccessive rounds of iterations.

FIG. 10 shows comparative performance improvement, i.e. improvement inprocess window, corresponding to various SRAF solutions. The X-axis inFIG. 10 represents DOF values, and the Y-axis represents exposurelatitude (%) values. Plot 1001 shows the DOF-EL curve for an SRAFsolution based on an initial SGM (before the iterative process of thepresent invention starts), and without corner consideration. Plot 1002shows the DOF-EL curve for an SRAF solution based on an initial SGM(before the iterative process of the present invention starts), and withcorner consideration. The DOF improvement between plots 1001 and 1002 isnot significant. However, the DOF values are noticeably improved after9^(th) iteration. Plot 1003 shows the result after 9^(th) iteration,with corner consideration (i.e., SRAF solution shown in FIG. 9C), andplot 1004 shows the result after 10^(th) iteration, with additionalcorner consideration (i.e., SRAF solution shown in FIG. 9E). DOFimprovement between plots 1002 and 1003 is significant (about 10 nm at5% EL). But DOF improvement between plots 1003 and 1004 is notsignificant. Therefore, the additional corner SRAFs 918 and 920 inlayout 900E do not contribute significantly to performance improvement.Accordingly, the 9^(th) iteration solution may be selected as theoptimum solution instead of the 10^(th) iteration solution, and theiterative process may be terminated after the 10^(th) iteration.

So far, placement of SRAFs around a relatively isolated square targetpattern 202 has been described. Target pattern 202 has enough roomaround it for placing SRAFs. Target pattern 202 may be part of an array,but the array does not have a very tight pitch in either Cartesiandirection. FIGS. 11A-B depict a much larger mask layout, where the pitchin Y-direction is very tight. Thus SRAF bars 1105 parallel to the Y-axisare placed in between successive contact pads in X-direction. Other SRAFpatterns are also placed around the contact pads.

FIG. 11A shows an example patch-boundary layout, where dashed boundarylines 1101 and 1102 demarcate four patches 1150, 1160, 1170, and 1180.Region 1104 within the dashed circular outline shows inconsistencies inSRAF placement (e.g., SRAF scatter bards are discontinuous) due to weaksignal strength that can not generate an optimal SGM for the entirelayout. A cutline 1103 is placed within the inconsistent region 1104 toprove the efficacy of the iterative optimization process of the presentinvention. In FIG. 11B, it is shown that after 10 short-loop iterations,an SRAF solution is obtained which eliminates the inconsistencies withinregion 1104 and elsewhere in the layout by boosting signal strength forproducing a better optimized SGM, resulting in better imaging.

FIG. 12 shows comparative plots of SRAF signal along the cutline 1103shown in FIG. 11A. Signal profile 1201 corresponds to initial SGM beforethe iterative optimization started. Within illustrative regions 1210 and1212 (within the dashed and dash-dotted outlines respectively), thesignal slope of profile 1201 is not discernable enough to place SRAFswith edge accuracy. However, signal profiles 1202, 1203 and 1204progressively improve signal strength as a result of successiveiterations. For 3^(rd) and 4^(th) iterations, signal strength variationis prominent enough (as shown by signal profiles 1203 and 1204respectively) to clearly identify a ridge location where a SRAF shouldbe placed.

FIG. 13 shows an array 1300 of contact pad target patterns 202 withcorresponding SRAFs placed in the vicinity of each pattern 202. It is tobe noted that SRAFs corresponding to each target pattern 202 do not needto be exactly similar. For example, in the array element 1302, thetarget pattern 202 a has a symmetric set of SRAFs (4 inner scatter barsand 4 outer scatter bars). However, in the array element 1304, thetarget pattern 202 b has an asymmetric set of SRAFs (4 inner scatterbars, 4 intermediate scatter bars, and one outermost scatter bar on theright side only). The asymmetry may be associated with one or moretarget patterns 202 placed at an edge of array 1300. As discussedbefore, persons skilled in the art will appreciate that though aplurality of SRAFs are shown around each target pattern 202, the scopeof the invention is not limited by the number of SRAFs. An optimized“set” of SRAF may include just one SRAF.

Details of a Computer System for Implementing the Embodiments of thePresent Invention

FIG. 14 is a block diagram that illustrates a computer system 100 whichcan assist and/or implement the SGM-optimization methods forlithographic simulation disclosed herein. Computer system 100 includes abus 102 or other communication mechanism for communicating information,and a processor 104 (or multiple processors 104 and 105) coupled withbus 102 for processing information. Computer system 100 also includes amain memory 106, such as a random access memory (RAM) or other dynamicstorage device, coupled to bus 102 for storing information andinstructions to be executed by processor 104. Main memory 106 also maybe used for storing temporary variables or other intermediateinformation during execution of instructions to be executed by processor104. Computer system 100 further includes a read only memory (ROM) 108or other static storage device coupled to bus 102 for storing staticinformation and instructions for processor 104. A storage device 110,such as a magnetic disk or optical disk, is provided and coupled to bus102 for storing information and instructions.

Computer system 100 may be coupled via bus 102 to a display 112, such asa cathode ray tube (CRT) or flat panel or touch panel display fordisplaying information to a computer user. An input device 114,including alphanumeric and other keys, is coupled to bus 102 forcommunicating information and command selections to processor 104.Another type of user input device is cursor control 116, such as amouse, a trackball, or cursor direction keys for communicating directioninformation and command selections to processor 104 and for controllingcursor movement on display 112. This input device typically has twodegrees of freedom in two axes, a first axis (e.g., x) and a second axis(e.g., y), that allows the device to specify positions in a plane. Atouch panel (screen) display may also be used as an input device.

According to one embodiment of the invention, portions of the simulationprocess may be performed by computer system 100 in response to processor104 executing one or more sequences of one or more instructionscontained in main memory 106. Such instructions may be read into mainmemory 106 from another computer-readable medium, such as storage device110. Execution of the sequences of instructions contained in main memory106 causes processor 104 to perform the process steps described herein.One or more processors in a multi-processing arrangement may also beemployed to execute the sequences of instructions contained in mainmemory 106. In alternative embodiments, hard-wired circuitry may be usedin place of or in combination with software instructions to implementthe invention. Thus, embodiments of the invention are not limited to anyspecific combination of hardware circuitry and software.

The term “computer-readable medium” as used herein refers to any mediumthat participates in providing instructions to processor 104 forexecution. Such a medium may take many forms, including but not limitedto, non-volatile media, volatile media, and transmission media.Non-volatile media include, for example, optical or magnetic disks, suchas storage device 110. Volatile media include dynamic memory, such asmain memory 106. Transmission media include coaxial cables, copper wireand fiber optics, including the wires that comprise bus 102.Transmission media can also take the form of acoustic or light waves,such as those generated during radio frequency (RF) and infrared (IR)data communications. Common forms of computer-readable media include,for example, a floppy disk, a flexible disk, hard disk, magnetic tape,any other magnetic medium, a CD-ROM, DVD, any other optical medium,punch cards, paper tape, any other physical medium with patterns ofholes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip orcartridge, a carrier wave as described hereinafter, or any other mediumfrom which a computer can read.

Various forms of computer readable media may be involved in carrying oneor more sequences of one or more instructions to processor 104 forexecution. For example, the instructions may initially be borne on amagnetic disk of a remote computer. The remote computer can load theinstructions into its dynamic memory and send the instructions over atelephone line using a modem. A modem local to computer system 100 canreceive the data on the telephone line and use an infrared transmitterto convert the data to an infrared signal. An infrared detector coupledto bus 102 can receive the data carried in the infrared signal and placethe data on bus 102. Bus 102 carries the data to main memory 106, fromwhich processor 104 retrieves and executes the instructions. Theinstructions received by main memory 106 may optionally be stored onstorage device 110 either before or after execution by processor 104.

Computer system 100 also preferably includes a communication interface118 coupled to bus 102. Communication interface 118 provides a two-waydata communication coupling to a network link 120 that is connected to alocal network 122. For example, communication interface 118 may be anintegrated services digital network (ISDN) card or a modem to provide adata communication connection to a corresponding type of telephone line.As another example, communication interface 118 may be a local areanetwork (LAN) card to provide a data communication connection to acompatible LAN. Wireless links may also be implemented. In any suchimplementation, communication interface 118 sends and receiveselectrical, electromagnetic or optical signals that carry digital datastreams representing various types of information.

Network link 120 typically provides data communication through one ormore networks to other data devices. For example, network link 120 mayprovide a connection through local network 122 to a host computer 124 orto data equipment operated by an Internet Service Provider (ISP) 126.ISP 126 in turn provides data communication services through theworldwide packet data communication network, now commonly referred to asthe “Internet” 128. Local network 122 and Internet 128 both useelectrical, electromagnetic or optical signals that carry digital datastreams. The signals through the various networks and the signals onnetwork link 120 and through communication interface 118, which carrythe digital data to and from computer system 100, are exemplary forms ofcarrier waves transporting the information.

Computer system 100 can send messages and receive data, includingprogram code, through the network(s), network link 120, andcommunication interface 118. In the Internet example, a server 130 mighttransmit a requested code for an application program through Internet128, ISP 126, local network 122 and communication interface 118. Inaccordance with the invention, one such downloaded application providesfor the illumination optimization of the embodiment, for example. Thereceived code may be executed by processor 104 as it is received, and/orstored in storage device 110, or other non-volatile storage for laterexecution. In this manner, computer system 100 may obtain applicationcode in the form of a carrier wave.

Example Lithography Tool

FIG. 15 schematically depicts an exemplary lithographic projectionapparatus for which lithographic processing can be simulated utilizingthe process of present invention. The apparatus comprises:

-   -   a radiation system Ex, IL, for supplying a projection beam B of        radiation. In this particular case, the radiation system also        comprises a radiation source SO;    -   a first object table (mask table) MT provided with a mask holder        for holding a mask MA (e.g., a reticle), and connected to first        positioning means PM for accurately positioning the mask with        respect to projection system PS;    -   a second object table (substrate table) WT provided with a        substrate holder for holding a substrate W (e.g., a        resist-coated silicon wafer), and connected to second        positioning means PW for accurately positioning the substrate        with respect to projection system PS;    -   a projection system (“lens”) PS (e.g., a refractive, catoptric        or catadioptric optical system) for imaging an irradiated        portion of the mask MA onto a target portion C (e.g., comprising        one or more dies) of the substrate W.

As depicted herein, the apparatus is of a transmissive type (i.e., has atransmissive mask). However, in general, it may also be of a reflectivetype, for example (with a reflective mask). Alternatively, the apparatusmay employ another kind of patterning means as an alternative to the useof a mask; examples include a programmable mirror array or LCD matrix.

The source SO (e.g., a mercury lamp or excimer laser) produces a beam ofradiation. This beam is fed into an illumination system (illuminator)IL, either directly or after having traversed conditioning means, suchas a beam expander or beam delivery system BD, for example. Theilluminator IL may comprise adjusting means AD for setting the outerand/or inner radial extent (commonly referred to as σ-outer and σ-inner,respectively) of the intensity distribution in the beam. In addition, itwill generally comprise various other components, such as an integratorIN and a condenser CO. In this way, the beam B impinging on the mask MAhas a desired uniformity and intensity distribution in itscross-section.

It should be noted with regard to FIG. 15 that the source SO may bewithin the housing of the lithographic projection apparatus (as is oftenthe case when the source SO is a mercury lamp, for example), but that itmay also be remote from the lithographic projection apparatus, theradiation beam that it produces being led into the apparatus (e.g., withthe aid of suitable directing mirrors); this latter scenario is oftenthe case when the source SO is an excimer laser (e.g., based on KrF, ArFor F₂ lasing). The current invention encompasses at least both of thesescenarios.

The beam B subsequently intercepts the mask MA, which is held on a masktable MT. Having traversed the mask MA, the beam B passes through thelens PS, which focuses the beam PS onto a target portion C of thesubstrate W. With the aid of the second positioning means (andinterferometric measuring means IF), the substrate table WT can be movedaccurately, e.g. so as to position different target portions C in thepath of the beam B. Similarly, the first positioning means can be usedto accurately position the mask MA with respect to the path of the beamB, e.g., after mechanical retrieval of the mask MA from a mask library,or during a scan. In general, movement of the object tables MT, WT willbe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which are not explicitlydepicted in FIG. 15. However, in the case of a wafer stepper (as opposedto a step-and-scan tool) the mask table MT may just be connected to ashort stroke actuator, or may be fixed.

Patterning device MA and substrate W may be aligned using alignmentmarks M1, M2 in the patterning device, and alignment marks P1, P2 on thewafer, as required.

The depicted tool can be used in two different modes:

-   -   In step mode, the mask table MT is kept essentially stationary,        and an entire mask image is projected in one go (i.e., a single        “flash”) onto a target portion C. The substrate table WT is then        shifted in the x and/or y directions so that a different target        portion C can be irradiated by the beam B;    -   In scan mode, essentially the same scenario applies, except that        a given target portion C is not exposed in a single “flash”.        Instead, the mask table MT is movable in a given direction (the        so-called “scan direction”, e.g., the y direction) with a speed        v, so that the projection beam PB is caused to scan over a mask        image; concurrently, the substrate table WT is simultaneously        moved in the same or opposite direction at a speed V=Mv, in        which M is the magnification of the lens PL (typically, M=¼ or        ⅕). In this manner, a relatively large target portion C can be        exposed, without having to compromise on resolution.

The concepts disclosed herein may simulate or mathematically model anygeneric imaging system for imaging sub wavelength features, and may beespecially useful with emerging imaging technologies capable ofproducing wavelengths of an increasingly smaller size. Emergingtechnologies already in use include DUV (deep ultra violet) lithographythat is capable of producing a 193 nm wavelength with the use of an ArFlaser, and even a 157 nm wavelength with the use of a Fluorine laser.Moreover, EUV lithography is capable of producing wavelengths within arange of 20-5 nm by using a synchrotron or by hitting a material (eithersolid or a plasma) with high energy electrons in order to producephotons within this range. Because most materials are absorptive withinthis range, illumination may be produced by reflective mirrors with amulti-stack of Molybdenum and Silicon. The multi-stack mirror has a 40layer pairs of Molybdenum and Silicon where the thickness of each layeris a quarter wavelength. Even smaller wavelengths may be produced withX-ray lithography. Typically, a synchrotron is used to produce an X-raywavelength. Since most material is absorptive at x-ray wavelengths, athin piece of absorbing material defines where features would print(positive resist) or not print (negative resist).

While the concepts disclosed herein may be used for imaging on asubstrate such as a silicon wafer, it shall be understood that thedisclosed concepts may be used with any type of lithographic imagingsystems, e.g., those used for imaging on substrates other than siliconwafers.

Although the present invention has been particularly described withreference to the preferred embodiments thereof, it should be readilyapparent to those of ordinary skill in the art that changes andmodifications in the form and details may be made without departing fromthe spirit and scope of the invention. It is intended that the appendedclaims encompass such changes and modification.

The invention may be further described using the following clauses:

1. A method for enhancing signal strength for placing sub-resolutionassist features (“SRAF”) with respect to one or more target patterns ina mask layout, comprising:

-   -   generating an initial SRAF guidance map for the mask layout;    -   placing a first set of one or more SRAF in the mask layout        according to the initial SRAF guidance map;    -   altering one or more of number, location and dimension of at        least a portion of the first set of one or more SRAFs;    -   measuring improvement in signal strength in the initial SRAF        guidance map; and    -   generating an updated SRAF guidance map with enhanced signal        strength.        2. The method of clause 1, wherein the method further comprises:    -   placing a subsequent set of one or more SRAF in the mask layout        according to the updated SRAF guidance map.        3. The method of clause 2, wherein the method further comprises:    -   repeating the steps of generating an updated SRAF guidance map        and placing the subsequent set of one or more SRAF in an        iterative process, until a predefined lithographic process        window criterion is satisfied.        4. The method of clause 3, wherein the step of iteratively        placing a subsequent set of one or more SRAF comprises:    -   replacing a prior set of one or more SRAF with a completely new        subsequent set of one or more SRAF.        5. The method of clause 3, wherein the step of iteratively        placing a subsequent set of one or more SRAF comprises:    -   retaining at least portions of a prior set of one or more SRAF;        and    -   adjusting the prior set of one or more SRAF to obtain the        subsequent set of one or more SRAF.        6. The method of clause 1, wherein the method further comprises:    -   applying optical proximity corrections (OPC) to original target        patterns in the mask layout to generate a corrected mask layout.        7. The method of clause 3, wherein the iterative process        comprises:    -   using a prior set of one or more SRAF and a prior corrected mask        layout to generate an updated SRAF guidance map;    -   using the updated SRAF guidance map to generate a subsequent set        of one or more SRAF;    -   applying OPC to the prior corrected mask layout to generate a        subsequent corrected mask layout that includes the subsequent        set of one or more SRAF; and    -   using the subsequent corrected mask layout to determine whether        the predefined lithographic process window criterion is        satisfied.        8. The method of clause 3, wherein the iterative process        comprises:    -   using a prior set of one or more SRAF and original target        patterns to generate an updated SRAF guidance map;    -   using the updated SRAF guidance map to generate a subsequent set        of one or more SRAF;    -   generating a subsequent mask layout that includes the subsequent        set of one or more SRAF; and    -   using the subsequent mask layout to determine whether the        predefined lithographic process window criterion is satisfied.        9. The method of clause 1, further comprising generating SRAF        placement rules using the SRAF guidance map.        10. The method of clause 1, wherein the mask layout includes a        predefined mask bias.        11. The method of clause 3, wherein the process window comprises        one or more of a focus window, and an exposure dose window.        12. The method of clause 3, wherein the iterative process is        terminated when a predefined cost function representing a        lithographic response reaches an optimum value that is        associated with the predefined process window criterion.        13. The method of clause 12, wherein the predefined cost        function represents an image log slope (ILS), an edge placement        error, a mask error enhancement factor, or a combination        thereof.        14. A computer program product comprising a computer-readable        medium having instructions recorded therein, which when        executed, cause the computer to generate files corresponding to        a mask layout having a plurality of target patterns to be imaged        in a lithographic imaging process, the generation of the files        comprising the steps of:    -   generating an initial SRAF guidance map for the mask layout;    -   placing a first set of one or more SRAF in the mask layout        according to the initial SRAF guidance map;    -   generating an updated SRAF guidance map using the first set of        one or more SRAF and the plurality of target patterns;    -   placing a subsequent set of one or more SRAF in the mask layout        according to the updated SRAF guidance map; and    -   repeating the steps of generating an updated SRAF guidance map        and placing the subsequent set of one or more SRAF in an        iterative process, until a predefined lithographic process        window criterion is satisfied.        15. The computer program product of clause 14, wherein the        method further comprises:    -   applying optical proximity corrections (OPC) to original target        patterns in the mask layout to generate a corrected mask layout.        16. The computer program product of clause 15, wherein the        iterative process comprises:    -   using a prior set of one or more SRAF and a prior corrected mask        layout to generate an updated SRAF guidance map;    -   using the updated SRAF guidance map to generate a subsequent set        of one or more SRAF;    -   applying OPC to the prior corrected mask layout to generate a        subsequent corrected mask layout that includes the subsequent        set of one or more SRAF; and    -   using the subsequent corrected mask layout to determine whether        the predefined lithographic process window criterion is        satisfied.        17. The computer program product of clause 14, wherein the        iterative process comprises:    -   using a prior set of one or more SRAF and original target        patterns to generate an updated SRAF guidance map;    -   using the updated SRAF guidance map to generate a subsequent set        of one or more SRAF;    -   generating a subsequent mask layout that includes the subsequent        set of one or more SRAF; and    -   using the subsequent mask layout to determine whether the        predefined lithographic process window criterion is satisfied.        18. The computer program product of clause 14, further        comprising generating SRAF placement rules using the SRAF        guidance map.        19. The computer program product of clause 14, wherein the mask        layout includes a predefined mask bias.        20. The computer program product of clause 14, wherein the        process window comprises one or more of a focus window, and an        exposure dose window.        21. The computer program product of clause 14, wherein the        iterative process is terminated when a predefined cost function        representing a lithographic response reaches an optimum value        that is associated with the predefined process window criterion.        22. The computer program product of clause 21, wherein the        predefined cost function represents an image log slope (ILS), an        edge placement error, a mask error enhancement factor, or a        combination thereof.        23. The computer program product of clause 14, wherein the step        of iteratively generating an updated SRAF guidance map        comprises:    -   altering one or more of number, location and dimension of at        least a portion of a prior set of one or more SRAFs; and    -   measuring improvement in signal strength to update a prior SRAF        guidance map.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described.

The descriptions above are intended to be illustrative, not limiting,Thus, it will be apparent to those skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

What is claimed is:
 1. A method implemented by a computer for placingsub-resolution assist features (“SRAF”) with respect to one or moretarget patterns in an original mask layout, comprising: generating aninitial SRAF guidance map for the original mask layout; generating afirst set of one or more SRAFs according to the initial SRAF guidancemap; generating a first altered mask layout corresponding to theoriginal mask layout by placing the first set of one or more SRAFs inthe original mask layout; determining, by the computer, a lithographicresponse for the first altered mask layout as a function of one or moreprocess window parameters; and determining whether the lithographicresponse for the first altered mask layout accommodates a predefinedprocess window size.
 2. The method of claim 1, wherein the methodfurther comprises: generating an updated SRAF guidance map using thefirst altered mask layout; generating a subsequent set of one or moreSRAFs having enhanced signal strength according to the updated SRAFguidance map; and generating a subsequent altered mask layout by placingthe subsequent set of one or more SRAFs in the original mask layoutaccording to the updated SRAF guidance map.
 3. The method of claim 2,wherein the method further comprises: repeating the steps of generatingan updated SRAF guidance map, generating the subsequent altered masklayout, and determining the lithographic response for the subsequentaltered mask layout in an iterative process, until the predefinedprocess window size is accommodated.
 4. The method of claim 3, whereinthe step of iteratively placing a subsequent set of one or more SRAFcomprises: replacing a prior set of one or more SRAF with a completelynew subsequent set of one or more SRAF.
 5. The method of claim 3,wherein the step of iteratively placing a subsequent set of one or moreSRAF comprises: retaining at least portions of a prior set of one ormore SRAF; and adjusting the prior set of one or more SRAF to obtain thesubsequent set of one or more SRAF.
 6. The method of claim 5, whereinadjusting the prior set includes altering one or more of number,location and dimension of the prior set of one or more SRAFs.
 7. Themethod of claim 3, wherein the iterative process comprises: applying OPCto the subsequent altered mask layout to generate a subsequent correctedmask layout that includes the subsequent set of one or more SRAF; andusing the subsequent corrected mask layout to determine whether thepredefined process window size is accommodated.
 8. The method of claim3, wherein the predefined process window size comprises one or more of afocus window size, and an exposure dose window size.
 9. The method ofclaim 3, wherein the iterative process is terminated when a predefinedcost function representing the lithographic response reaches an optimumvalue that is associated with the predefined process window size. 10.The method of claim 1, wherein the method further comprises: applyingoptical proximity corrections (OPC) to original target patterns in themask layout to generate a corrected mask layout.
 11. The method of claim1, further comprising generating SRAF placement rules using the SRAFguidance map.
 12. The method of claim 1, wherein the mask layoutincludes a predefined mask bias.
 13. The method of claim 1, wherein thelithographic response is one or more of an image log slope (ILS), anedge placement error, and a mask error enhancement factor.
 14. Themethod of claim 1, further comprising: measuring improvement in signalstrength in the initial SRAF guidance map; and generating an updatedSRAF guidance map with enhanced signal strength.
 15. A computer programproduct comprising a non-transitory computer-readable medium havinginstructions recorded thereon, which when executed, cause the computerto perform a method for placing sub-resolution assist features (“SRAF”)with respect to one or more target patterns in a an original masklayout, the method comprising: generating an initial SRAF guidance mapfor the original mask layout; generating a first set of one or moreSRAFs according to the initial SRAF guidance map; generating a firstaltered mask layout corresponding to the original mask layout by placingthe first set of one or more SRAFs in the original mask layout;determining, by the computer, a lithographic response for the firstaltered mask layout as a function of one or more process windowparameters; and determining whether the lithographic response for thefirst altered mask layout accommodates a predefined process window size.